The vehicle bearing apparatus, such as an automobile, that rotatably supports the wheel relative to a suspension apparatus, via double row rolling bearing, is of two types, one for a driving wheel and one for a driven wheel. As one representative example of a structure of the prior art, a vehicle bearing apparatus for a driving wheel is shown in FIG. 3. In the description below, the term “outboard side” (left-hand side in drawings) of the apparatus denotes a side that is positioned outside of the vehicle body. The term “inboard side” (right-hand side in drawings) of the apparatus denotes a side that is positioned inside of the body when the bearing apparatus is mounted on the vehicle body.
The bearing apparatus is a so called second generation and has a wheel hub 60 and a wheel bearing 61. The wheel hub 60 has, at one end, an integrally formed wheel mounting flange 62. A cylindrical portion 60a extends axially from the wheel mounting flange 62. The wheel bearing 61 is press fit onto the cylindrical portion 60a via a predetermined interface.
The wheel bearing 61 has an outer member 63 with a body mounting flange 63b on its outer circumferential surface and double row tapered outer raceway surfaces 63a, 63a on its inner circumference surface. A pair of inner rings 64, 64 is inserted into the outer member 63. The inner rings are formed with tapered inner raceway surfaces 64a on their outer circumferential surfaces. The tapered inner raceway surfaces 64a are arranged opposite to the double row outer raceway surfaces 63a, 63a. Double row tapered rollers 65, 65 are contained between the inner and outer raceway surfaces 64a, 63a. Cages 66 rollably hold the double row tapered rollers 65, 65 in place.
The wheel bearing apparatus of the second generation type has advantages of enabling standardization of the wheel bearing 61 and reducing manufacturing cost due to its mass production. On the contrary, there is a desired not only for the reduction of manufacturing cost but for reduction of weight and size of the wheel bearing apparatus. Thus, a wheel bearing apparatus of a third generation type, as shown in FIG. 4, has been proposed. It has a wheel hub and a wheel bearing with a double row rolling bearing formed as a unit.
This third generation type wheel bearing apparatus has a wheel hub 51, an inner ring 52, an outer ring 53 and double row balls 54, 54. The wheel hub 51 has an integrally formed wheel mounting flange 55 on its one end. On its outer circumferential surface, it includes an inner raceway surface 51a. A cylindrical portion 51b extends axially from the surface 51a. Hub bolts 56, for securing a wheel (not shown) on the flange 55, are arranged equidistantly along the periphery of the flange 55.
The inner ring 52 is formed with an inner raceway surface 52a, of the inner side, on its outer circumferential surface. The inner ring 52 is press fit onto the cylindrical portion 51b of the wheel hub 51. The inner ring 52 is axially immovably secured by a caulking portion 51 to prevent it from slipping off of the cylindrical portion 51b. The caulked portion 51c is formed by radially outwardly plastically deforming the end portion of the cylindrical portion 51b. 
The outer ring 53 is integrally formed with a body mounting flange 53b on its outer circumferential surface. Also, it is formed with double row outer raceway surfaces 53a, 53a on its inner circumferential surface. Double row rolling elements 54, 54 are freely rollably contained between the opposing outer raceway surfaces 53a, 53a and the inner raceway surfaces 51a, 52a. 
The wheel hub 51 is made of carbon steel including carbon of 0.40˜0.80% by weight. It is formed with a hardened layer (shown by cross-hatching) in a region from the base of the wheel mounting flange 55 to the cylindrical portion 51b through the inner raceway surface 51a. The hardened layer is formed by high frequency induction hardening. The caulked portion 51c remains as its original surface hardness after forging. The inner ring 52 is made of high carbon chrome bearing steel such as SUJ2 and is hardened to its core by quenching.
Thus, it is possible to realize a vehicle wheel bearing apparatus with low cost that has sufficient durability, prevents the generation of damage such as cracks on the caulked portion 51c, and also prevents the generation of large change in the diameter of the inner ring 52 secured on the wheel hub 51 by the caulked portion 51c. Also, it is possible to reduce the potential for the inner ring 52 to be damaged by the securing work, to maintain the pre-load at a appropriate value, and also to reduce the number of parts and steps of machining and assembly process (see Japanese Laid-open Patent Publication No. 129703/1999).